This invention relates to a method for removing liquid from industrial wastewater sludge and, more particularly, to a method for removing liquid from chemically-precipitated sludge prior to disposal.
Industrial wastewater sludge consists principally of a mixture of solid phases suspended in an aqueous solution of dissolved substances. Sludge of this type is normally processed prior to disposal for the purpose of concentrating the sludge solids and reducing the liquid content of the sludge. Processing of industrial wastewater sludge to reduce its liquid content is commonly referred to as "dewatering". Sludge dewatering is an economic necessity in order to reduce the volume of sludge requiring disposal. Recently, sludge dewatering has come to be regarded as ecologically desirable as well. Legislation passed in a number of jurisdictions now requires that a specific minimum solids content level be met before sludge will be accepted for landfill disposal. The purpose of this legislation is to maintain at a safe level the amount of potentially harmful leachate emanating from landfill sites. In some jurisdictions, a solids content as high as 30 percent is required before sludge disposal in landfills will be permitted.
As a practical matter, it is virtually impossible to obtain sludge meeting these high solids content requirements by prior art dewatering techniques wherein concentration of the sludge solids is carried out in settling lagoons, holding tanks, or similar containment means in which the sludge solids settle by gravitational force.
The presently available mechanical methods for dewatering sludge, such as by vacuum filtration or centrifugation, while capable of producing sludge having a solids content of greater than 30 percent, involve the use of specialized equipment which requires a substantial investment for installation, operaton, maintenance and repair. Further, the known dewatering methods which involve passage of an electric current through the sludge have not proved to be particularly efficient. In general, electrical dewatering occurs at the cathode only. Sludge solids accumulate on the anode in a relatively short period of time and terminate the flow of electric current in, and the removal of liquid from the system, thus necessitating the interruption of the process while the accumulated solids are removed from the anode, or the provison of means for continuously removing the treated sludge from the treatment vessel. Representative prior art dewatering methods involving electrical treatment of the sludge are described in U.S. Pat. Nos. 3,664,940 and 3,962,069. These prior are electrical treatment processes require complex apparatus which contributes significantly to their overall expense. Such unfavorable economic factors have tended to seriously hinder the acceptance and use of electrical sludge treatment on a commercial basis.
Hence, the expense of mechanical and electrical dewatering apparatus makes it uneconomic for many manufacturing plant opertors who have a wastewater disposal problem, and who might advantageously employ an on-site sludge dewatering system.
The development of efficient and effective methods for dewatering industrial wastewater sludge continues to be a highly desired objective.